Apparent synthesis of thiamin and vitamin B12 in rumen of lactating dairy cows fed alfalfa or orchardgrass silages at different maturity stages
Effects of maturity stage of forages on apparent ruminal synthesis and post-ruminal supply of thiamin and vitamin B12 were evaluated in two experiments. Alfalfa and orchardgrass were harvested and ensiled as a) early-cut, less mature (EC) and b) late-cut, more mature (LC). Diets containing alfalfa or orchardgrass silages of each maturity stage as the sole forage were offered to ruminally and duodenally cannulated lactating Holstein cows in crossover design experiments. Experiment 1 compared diets containing EC and LC alfalfa silage (~22% forage NDF and ~27% total NDF) offered to 16 cows in two 17-d treatment periods. Experiment 2 compared diets containing EC and LC orchardgrass silage (~25% forage NDF and ~30% total NDF) offered to 13 cows in two 18-d treatment periods. Intakes of thiamin increased with maturity stage of forages in Experiment 1 (P<0.01; 79.3 vs. 46.5 ± 2.08 mg/d) and in Experiment 2 (P<0.01; 76.0 vs. 60.9 ± 1.84 mg/d for LC and EC, respectively). In contrast, intakes of vitamin B12 were lower for LC than EC in Experiment 1 (P<0.01; 191 vs. 329 ± 8.47 µg/d) and in Experiment 2 (P<0.01; 53.1 vs. 76.3 ± 1.71 µg/d). In Experiment 1, duodenal flows of thiamin were greater for LC than EC (P=0.02; 156 vs. 130 ± 7.21 mg/d) but the duodenal flows of vitamin B12 were not affected by treatment (P=0.21, 12182 ± 556.1 µg/d). In Experiment 2, duodenal flows of vitamins were not affected by treatment (thiamin, P=0.20, 199 ± 8.47 mg/d; vitamin B12, P=0.28, 8518 ± 426.0 µg/d). The apparent ruminal synthesis of thiamin and vitamin B12 were not affected by treatment in either experiment (Experiment 1: thiamin, P=0.45, 79.8 ± 6.56 mg/d; vitamin B12, P=0.28, 11924 ± 426.0 µg/d; Experiment 2: thiamin, P=0.18, 130 ± 8.57 mg/d; vitamin B12, P=0.29, 8454 ± 426.0 µg/d). Delaying the harvest of alfalfa or grass resulted in a greater dietary supply of thiamin and a lower supply of vitamin B12. Nevertheless, the stage of maturity did not affect apparent ruminal synthesis and had little or no effect on the post-ruminal supply of these vitamins.
Keywords: dairy cow, thiamin, vitamin B12